Manganese-nickel-aluminum-copper-molybdenum system age-hardenable steel for plastic molds

ABSTRACT

THE DISCLOSED STEELS FOR USE AS MATERIALS FOR PLASTIC MOLDS CONSIST ESSENTIALLY OF 0.05-0.18% C, 0.15-1.0% SI, 1.0-2% MN, 2.5-3.5% NI, 0.5-1.5% AI, 0.7-1.7% CU, 0.1-0.4% MO AND THE BALANCE, IRON AND ARE AGE-HARDENABLE AT ABOUT 500* C. FOR ABOUT FIVE HOURS AFTER BUILD-UP   WELDING. TO IMPROVE THE MACHINABILITY, TOUGHNESS AND GRAIN REFINING PROPERTY, THE STEELS MAY BE ADDED, FOR EXAMPLE WITH UP TO 0.3% S, UP TO 2.5% CR AND UP TO 2.5% CR AND UP TO RESPCTIVELY.

3,824,096 -NICKEL-ALUMINUM-COPPER-HOLYBDENUM SYS AGE-HARDENABLE STEEL FOR PLASTIC lows TEME July 16, 1974 CHIAKI' ASADA ETAL MANGANESE Filed June 21, 1972 5 Shoots-Shoot 1 FIG.

FIG. 2 PRIOR ART m A TT OE I: M M 0 /l Z 9 m D T T E C C T L E E A F F T F F OE A A M m m m E E M H l m L mm m mm A R l m 3 E F MOTIHER METAL MOTHER HEAT A FFECTED ZONE METAL July 16, 1974 CHIAKI ASADA HAL. 3,824,096

MANGANESENICKEL-ALUHINUM-COPPER-"OLYBDENUH SYSTEM AGl-HARDENABLE STEEL FOR PLASTIC HOLDS 5 Shoots-Shoot 2 Filed June .21, 1972 EPOSITED METAL HEAT AFFECT MOTHER METAL MoTHE METAL FIG. 5

ED I ZONE HEATA'FFECTED zoNE WIDTH OF HEAT AFFECTED ZONE DECREASED IN HARDNESS IN mm ZEE mwIPOE n24 mzom Qw. .0wh n .cqwI zwwEwm wmwzomqI 7: IQ wozmmwmmfi FIG. 6

Mo CONTENT IN July 16, 1974 CHIAKI ASADA ETAL 3,324,096vv MANGANESE'NICKEL-ALUMINUMCOPPER-MOLYBDENUM SYSTEM AGE-HARDCNABLE STEEL FOR PLASTIC HOLDS '5 Sheets-Sheet 5 Filed June 21, 1972 FIG. 7

1 M as 2 m 9:? EE 85m: $162 93 omtmoawa m1. 255% wmmzomqz z. wwzwwmka C CONTENT IN FIG. 8

07% Cu I.O%Cu

AL CONTENT IN O U T V M & a 3 3 2 Z .Zm:.ZOQ Z July 16, 1974 CHIAKI ASADA 3,324,096

MANGANESE-NICKEL-ALUMINUM-GOPPEH-HOLYBDENUM SYSTEM AGE-HARDINABLE STEEL FOR PLASTIC HOLDS 5 Sheets-Sheet 4 Filed June 21, 1972 FIG. 9

mmmZQm I mmmzu O O O 0 FIG.

5 R H 5 W F -4 m -2 m D m A 8 S A 6 -4 2 0 2 -4 6 O 0 w w 4 w BOlUPIDARK DEPOSITED' MOTHER METAL 'STEAL DISTANCE FROM BOUNDARY BETWEEN DEPOSITED STEEL AND MOTHER METAL IN mm July 16, 1914 MANGANESE-NICKEL-ALUMINUM-COPPER-HOLYBDENUM SYSTEM Filed June 21, 1972 ROCKWELL HARDNESS (HRC) CHIAKI ASADA ETAL 3,324,096

AGE'HARDINABLE STEEL FOR PLASTIC HOLDS 5 Sheets-Sheet 5 FIG. II

INVENTION E /o o AISI 4145 AISI 1064 20 x x REHEATING TEMPERATURE IN "C United States Patent l US. Cl. 75-124 4 Claims ABSTRACT OF THE DISCLOSURE The disclosed steels for use as materials for plastic molds consist essentially of 0.050.18% C, 0.l5-l.0% Si, 1.0-2% Mn, 2.53.5% Ni, 0.51.5% Al, 0.7-1.7% Cu, 0.1-0.4% Mo and the balance, iron and are age-hardenable at about 500 C. for about five hours after build-up welding. To improve the machinability, toughness and grain refining property, the steels may be added, for example with up to 0.3% S, up to 2.5% Cr and up to 0.5% Ti respectively.

BACKGROUND OF THE INVENTION This invention relates to manganese-nickel-alu-minumcopper-molybdenum system age-hardenable steels for plastic molds having such a property that the deposited metal and heat affected zone resulting from build-up welding can be subject to uniform photoetching as easily as the associated mother metal through aging treatment. The invention is also concerned with such steels improved in at least one of the machinability, grindability, toughness, hardenability, and grain refining property.

Upon manufacturing plastic molds, the photoetching or chemically machining technique has been in almost all cases utilized photographically to form the anticorrosive film in a desired pattern on the internal surface of the mold. Heretofore, carbon steels and structural low-alloy steels have been generally employed to form plastic molds, but those steels have been difiicult to be excellent in all the characteristics presently required for materials of such molds. Those characteristics involve the machinability, grindability, ability to be polished into the mirror surface, susceptibility to photoetching, weldability, electrospark machinability, compressive strength, corrosion re sistance, wear resistance, dimensional stability etc. The existing mold steels have been very difiicult to include all those characteristics because some of the characteristics incompatible with one another, and some of them can not be inherently compromised.

On the other hand, plastic molds recently have a tendency to increase in hardness, because the reinforcing material such as glass fibers may be added to resins to be molded and also to become complicate in cavitys configuration to suit the particular requirements. Therefore during and after the die milling operation, the particular mold may not be avoided to be corrected or retouched. Build-up welding for correction conducted with mold steels high in hardness has caused the discontinuity of the hardness and photoetching property of the heat affected zone of the resulting weld part.

In order to uniformly photoetch those steels after having undergone build-up welding, it has been proposed and it is now being searched to alternately quench and temper them in repeated manner to render the weld part and heat affected zone equal in the structure to the mother metal. However it can be said that the weld part, heat affected zone and mother metal will be practically impossible to be uniformly photoetched with the existing steels for plastic 3,824,096 Patented July 16, 1974 molds. This leads to the necessity of eliminating buildup welding for correction purpose. Unfortunately, as patterns and/or figures on internal surfaces of molds tend to be more complicated, it becomes increasingly diflicult to eliminate the correction of the molds through build-up welding.

Also for conventional steels for plastic molds, repeating alternate quenching and tempering is the one and only means for uniformly photoetching the resulting built-up weld part. This is because such steels are low alloy steels transformable to the martensitic structure. However since build-up welding is generally conducted with molds having respective cavities nearly completed therein, the succeeding quenching and tempering can cause the oxidation and deformation of the cavities resulting in less effectiveness. Accordingly it is highly desirable to provide steels for plastic molds capable of being uniformly photoetched for build-up welding apparatus.

SUMMARY OF THE INVENTION Accordingly it is a general object of the invention to minimize or substantially eliminate the above mentioned disadvantages of the conventional steels for plastic molds.

It is an object of the invention to provide a new and improved age-hardenable steel of manganese-nickel-aluminum-copper-molybdenum system for plastic molds having a good machinability so that it can be worked into mold with a hardness age hardened to 40 or more on the Rockwell hardness C scale while the steel as build-up Welded can readily be machined into molds and capable of being age-hardened and subject to photoetching With no oxidation nor deformation.

It is another object of the invention to improve the machinability, grindability, the polishing property, susceptability to photoetching, weldability, electrospark machinability, compressive strength, corrosion resistance, wear resistance and/or dimensional stability etc. of the steel as described in the preceding paragraph.

The invention accomplishes these objects by the provision of an age-hardenable steel of manganese-nickelaluminum-copper-molybdenum system consisting essentially of from 0.05 to 0.18% by weight of carbon, from 0.15 to 1.0% by weight of silicon, from 1.0 to 2.0% by weight of manganese, from 2.5 to 3.5% by weight of nickel, from 0.5 to 1.5% by Weight of aluminum, from 0.7 to 1.7% by weight of copper, from 0.1 to 0.4% by weight of molybdenum, and the balance being iron and very small amounts of incidental impurities, the steel after having been welded being capable of aging; to provide a weld part in which the deposited metal and heat affected zone can readily be uniformly photoetched to the substantially same extent as does the mother metal.

In order to increase the machinability, the steel as above described may have added thereto at least one element selected from the group consisting of up to 0.3 by weight of sulfur, up to 0.4% by weight of lead, up to 0.5% by weight of selenium, up to 0.3% by weight of tellurium, and up to 0.3% by weight of bismuth.

In order to improve the toughness and hardenability, the steels may have added thereto at least one element selected from the group consisting of up to 2.5 by weight of chromium, up to 0.5% by weight of tungsten, up to 0.5% by weight of cobalt, up to 0.5% by weight of beryllium, and up to 0.01% by weight of boron.

In order to promote the refining of grains in the steel, there may be used at least one selected from the group consisting of up to 0.5% by weight of titanium, up to 0.5% by weight of vanadium, up to 0.3% of the sum of niobium and tantalum and up to 0.5% by Weight of zirconium.

3 BRIEF DESCRIPTION OF THE DRAWINGS Other objects, feature and advantages of the invention will be more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a longitudinal sectional view of a typical weld part;

FIG. 2 is a microscopic photograph of photoetched surface of a weld part formed of an existing steel for plastic molds with the surface lying in a plane as taken along the line IIII of FIG. 1;

FIG. 3 is a photograph similar to FIG. 2 but illustrating a conventional age-hardenable steel for plastic molds with the surface lying in a plane as taken along the line IIIIII of FIG. 1;

FIG. 4 is a photograph similar to FIG. 2 but illustrating the present invention with the photoetched surface also lying in the plane as taken along the line IIII of FIG. 1;

FIG. 5 is a scatter diagram illustrating the relationship between a difference in hardness between a heat affected zone and the associated mother metal and a width of a hardness decreased region of the heat affected zone of built-up weld part after a heat treatment;

FIG. 6 is a graph illustrating a width of that region of the heat affected zone in which a difference in hardness is below a predetermined value, as a function of a content of molybdenum;

FIG. 7 is a graph illustrating a difference in hardness between the deposited metal and mother metals as a function of a content of carbon after an aging treatment;

FIG. 8 is a scatter diagram illustrating proportions of nickel, aluminum and copper required to maintain a difference in hardness between the heat affected zone and mother metal at a predetermined value with the parameter being the content of carbon;

FIG. 9 is a graph plotting a hardness against a distance from an electrospark machined surface for the present steel;

FIG. 10 is a graph illustrating hardness distributions on a weld part as welded after having age-hardened; and

FIG. 11 is a graph illustrating a hardness plotted against a reheating temperature for the present and conventional steels with the reheating time maintained constant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIG. 1 in particular, there is typically illustrated a weld part formed by build-up welding. As shown in FIG. 1, a mother or parent metal 10 has deposited on one surface thereof a deposited metal 12 to form a heat affected zone 14 therebetween.

Weld parts such as shown in FIG. 1 were formed of one existing steel for use as the material of plastic molds called herein Type Y whose composition will be listed in the undermentioned Table I. A suitable milling machine was used to cut off the weld parts down to a plane such as shown at broken line IIII in FIG. 1, whereby the mother metal, heat affected zone and deposited metal were exposed in a common plane positioned at depth of 1 mm. from the surface of the mother metal of the weld part before machining. The machined weld parts were tempered in under a vacuum at 650 C. for three hours and then etched with an etching solution usually used in photoetching techniques well known in the art. The etched weld parts typically have the etched surface as shown in a microscopic photograph of FIG. 2. As clearly shown in FIG. 2, the etched surface exhibits a lack of uniformity on that portion thereof extending from the deposited metal to the boundary between the heat affected zone and the mother metal. This means that the mother metal is different in surface corrosion or roughness from the remaining portion of the weld part. That is, the etched surface has irregularities which will be, in turn, transferred to the surfaces of the resulting plastic moldings leading to defected products.

FIG. 3 is a microscopic photograph typically illustrating a conventional age-hardenable steel called herein Type A whose composition will be also listed in Table I. In that case, the weld parts were cut off down to a plane such as shown at broken line IIIIII in FIG. 1 positioned at a depth of 3 mm. from the surface of the original mother metal. As seen in FIG. 3, the exposed surface of the weld part thus machined included no deposited metal zone. The weld parts were then subject to an aging treatment at 500 C. for five hours followed by etching. FIG. 3 depicts that a lack of uniformity still remains in the etching surface of the heat affected zone.

The invention is based upon the discovery that the weld part is excellent in photoetching property as far as it is of a uniform metallugical structure and small in difference in hardness occurring from point to point on the surface thereof.

According to one aspect of the invention, there is provided a manganese-nickel-aluminum-copper-molybdenum system age-hardenable steel for plastic molds consisting essentially of from 0.05 to 0.18% by weight of carbon, from 0.15 to 1.0% by weight of silicon, from 1.0 to 2.0% by weight of manganese, from 2.5 to 3.5% by weight of nickel, from 0.5 to 1.5% by weight of aluminum, from 0.7 to 1.7% by weight of copper, from 0.1 to 0.4% by weight of molybdenum and the balance being iron and very small amounts of incidental impurities. After having aged, the steel can uniformly undergo to photoetching.

The present steels are enabled to be easily machined into molds after the steels have been age-hardened to a Rockwell hardness of about 40 or more on the C scale. Alternatively, the steels can readily be machined into molds even after built-up welding. Moreover, the resulting molds can be again age-hardened at a temperature in the order of 500 C. to permit the molds to be uniformly subject to photoetching with little change in dimension or in a strain due to heat treatments.

In order that a difference in hardness between the deposited metal and heat affected zones and the mother metal is equal to or less than a Rockwell hardness of about 2 on the C scale, the contents of the ingredients have been determined as above specified.

In the invention, carbon is effective for facilitating the formation of the martensitic or bainitic structure upon cooling at relatively high cooling rate from the temperature of solution heat treatment. However an excessive amount of carbon added deteriorates the machinability of the steels as solution heat treated and also decreases in toughness after the aging treatment. The content of carbon causes an increase in hardness difference as will be described hereinafter. It has been found that the content of carbon is required to range from 0.05 to 0.18% on ghel basis of the total weight of the steel as above speci- Silicon serves as an element for adjusting the hardness of the present steels as solution heat treated. With the steel used in a large mass, only the addition of manganese can not control the hardness thereof as solution heat treated so that the silicon should be added in an amount of from 0.15 to 1.0% by weight which is effective for preventing the steel from being impeded in both ductility and toughness after the aging treatment.

Manganese affects the hardness of the present steels as solution heat treated and as age-hardened. Also the manganese cooperates with the carbon to increase the hardenability and aging hardness of the steels upon cooling them from the temperature of solution heat treatment. It has been found that, in order to increase the aging hardness to a Rockwell hardness of at least about 40 on the C scale (or H CZ tO), the content of manganese should range from 1.0 to 2.0% on the basis of the total weight of the steel. A content of manganese less than 1.0% is less effective and an amount of manganese added in excess of 2.0% is not desirable in that the machinability and toughness is adversely affected.

One portion of nickel added to the present steel forms a solid solution with total amount of copper thereby to prevent the occurrence of a red shortness during hot working. In the solution treated state the nickel cooperates with the copper to form the 6 phase through a further aging treatment. The 6 phase provides nuclei about which the Ni-Al phase or a phase precipitates. Also the nickel is one of the indispensable ingredients for forming the a phase with aluminum after the aging treatment. Further, in order to ensure the good quality of photoetched surfaces as will be described hereinafter, the nickel is required to be included in the steel in an amount of from 2.5 to 3.5% on the basis of the total weight of the steel. A content of nickel outside the range just specified is less effective.

As above described, aluminum constitutes an indespensable ingredient for precipitating the Ni-Al phase with the nickel after the aging treatment. Also the aluminum is required to be added to the steel in an amount of at least 0.5% by weight for the purpose of retaining a good photoetching property. However an excessive amount of the aluminum added deteriorates the productivity, ability to be finished into the mirror surface, ductility and toughness. For this reason, the upper limit for the aluminum has been determined to be 1.5% on the base of the total weight of the steel. After having been age-hardened, steels including aluminum and nickel as age-hardening elements have the hardness dependent upon the content of aluminum.

As above described, copper plays an important role in forming the nuclei about which the a phase is precipitated in the aging state and more effective particularly for small contents of nickel and aluminum. It has been found that the copper is an indispensable alloying ingredient for improving the notch toughness through hot working. It has been also found that the copper is effecti-ve for improving the machinability of the steels as solution heat treated leading to the necessity of adding to the steels the copper in an amount of at least 0.7 weight percent. However it has been found that the addition of the copper in an amount exceeding 1.7% is disadvantageous in view of the hot shortness and economy. Accordingly, the content of copper is required to range from 0.7 to 1.7% on the basis of the total Weight of the steel.

Molybdenum is indispensable for the purpose of ensuring the excellence of the photoetching property. This results in the necessity of adding to the steels the molybdenum in an amount of at least 0.1 weight percent. If amounts of molybdenum added exceed 0.4 weight percent, the excellent photoetching property is not correrials. Thus the content of molybdenum has been determined to range from 0.1 to 0.4 weight percent.

According to another aspect of the invention, the steels as above described can be improved in any one or more of the machinability, toughness, hardenability, and grain refining property.

In order to improve the machinability, at least one element selected from the group consisting of sulfur, lead, selenium, tellurium and bismuth can be added to the steels. It has been found that any one or more of sulfur in an amount up to 0.3%, lead in an amount up to 0.4%, selenium in an amount up to 0.5%, tellurium in an amount up to 0.3% and bismuth in an amount up to 0.3% on the basis of the total Weight of the steel can be added to the steels as above described to remarkably improve the machinability. Any of these elements added in excess of the corresponding figure just specified adversely affects the ductility and toughness of the resulting steels.

If it is desired to form large molds of the steel as above described, then at least one element selected from the group consisting of up to 2.5% of chromium, up to 0.5% of tungsten, up to 0.5% of cobalt, up to 0.5% of beryllium and up to 0.01% of boron on the basis of the total weight of the alloy can be added to the present steel thereby to improve the toughness and hardenabi-lity of the resulting steels. The addition of any one or more of those elements also serves to control the hardness of the resulting steels as solution heat treated and as age hardened. The addition of any one of the elements Cr, W, Be and B in its amount in excess of the corresponding figure just specified causes increase in material cost and also is less effective.

Titanium, vanadium, niobium plus tantalum and zirconium are effective for refining grains of the resulting steel and increasing the toughness thereof as well as improving the notch toughness thereof. However the steels having added thereto any one of those elements in an excessive amount have the hardness is aging or solution heat treated state increased beyond the required value. It has been found that the satisfactory results are obtained with at least one selected from the group consisting of up to 0.5% of titanium, up to 0.5% of vanadium, up to 0. 3% of the sum of niobium and tantalum, and up to 0.5% of zirconium on the basis of the total Weight of the steel.

If desired, the steels may have added thereto those elements selected from at least two of the groups of the machinability improving elements, the group of the toughness and hardenability improving elements and the group of the grain refining elements as above described in order to improve the corresponding characteristics.

Examples of the invention are listed in the following Table I in which the conventional steels are also listed spondingly exhibited and increased in the costs of matefor control purposes.

TABLE I.COMPOSITIONS AND HARDNESSES OF PRESENT AND CONVENTIONAL STEELS Fundamental alloying ingredients in wt. percent Desirable alloying ingredients in wt. Hard- Type of steel 0 S1 Mn P Ni Al Cu Mo percent ness" See footnotes at end of table.

TABLE IContinued Fundamental alloying ingredients in wt. percent Desirable alloying ingredients in wt. Hard- Type of steel Si Mn P Ni Al Cu M0 percent ness 14 30 1. 39 011 3. 48 1. 53 51 36. 13 31 1. 50 012 3. 01 2.03 45 36. 5 .40 20 .75 016 15 30.1 .13 .21 1.51 .025 3.21 .91 .92 41.0 14 25 1. 45 019 3. 41 91 85 43. 5 10 .23 1. 54 025 3. 45 1. 30 1.10 42. 5 11 23 1. 61 020 3. 15 1.01 1. 15 41. 0 13 29 1. 61 018 3.05 1. 1. 20 44. 0 09 29 1. 35 018 3. 51 95 .98 45.0 11 21 1. 61 021 3. 20 95 .89 42. O .13 .30 1.60 .025 3.10 .87 1.05 .30 .25 W, .10 Ti 43.7 13 21 1. 41 025 3. 41 97 1.05 35 .57 Ct, .20 Nb+Ta 44.1 .09 .25 1.45 .019 3.13 ,92 1.07 .23 .13 Pb, .003 P, .13 TL- 39. 6 .08 .51 1.27 .017 3.51 .94 1.04 .21 .14 Pb, .75 Cr, .11 V-.. 40. 5

REMAnK.The symbol indicates the steels for control purpose. The symbol indicate the value on the 0 scale of Rockwell hardness (HRG) after having been age-hardened at 500 C In the above Table I, all steels aged at 500 C. for five hours except for the steel labelled Type Y being tempered at 650 C. for three hours. The present steels exhibited good phototeched surfaces whereas the control steels except for Types V and X steels were not good in photoetching property. The present steels are preferably agehardened at a temperature ranging from 450 to 550 C.

Weld parts formed of an age-hardenable steel of the invention labelled Type D in the above Table I were machine in the same manner as previously described in conjunction with FIG. 2 and similarly aged and etched. The resulting etched surface was substantially free from a lack of etching uniformity as shown in a microscopic photograph of FIG. 4. As shown in Table I, Type A steel forming the weld part illustrated in FIG. 2 included no molybdenum while Type D steel forming the weld part shown in FIG. 4 included 0.23% by weight of molybdenum. From this it will be appreciated that the presence of molybdenum greatly contributes to an improvement in the photoetching property of the present steels.

The results of numerous experiments have indicated that a difference in hardnees between the heat affected zone and the mother metal zone of built-up weld parts after having aged affects the quality of the etched surface thereof with the deposited metal identical in type of steel to the mother metal. As previously described in conjunction with FIGS. 1 and 2, built-up weld parts were formed of many different types of steel and machined followed by suitable aging treatment. Then the hardness was measured at various points on the surface of the weld part. Also those weld parts were etched with an etching solution commonly used in the photoetching process.

The results of the measurements are shown in FIG. 5 wherein the axis of ordinates represents a diflerence in hardness AH between the heat affected zone and the mother metal of the weld part on the C scale of Rockwell hardness and the axis of abscissas represents a width of that portion of the heat affected zone decreased in hardness. The width was measured in mm. perpendicularly to the longitudinal axis of the deposited metal. In FIG. 5, the symbol circle means that the etched surface is substantially uniform while the symbol dot in circle means that the etched surface is irregular. Also the reference character designated beside each symbol represents the type of steel of which the associated weld part is formed and has the same meaning as that listed in Table I. For example, the symbol circle labelled F described that weld part formed of Type F steel listed in Table I has been uniformly etched. In FIG. 5, those weld parts formed of Types A, B, G and H of the conventional steel are shown as lying outside of a hatched portion defined by curve a and being irregularly etched.

It has been found that, in order to ensure the uniformity of the photoetching property of weld parts after having aged, it is sufficient either to maintain the width d of the hardness decreased portion of the heat affected zone equal to or less than 1 mm. or to render the difference in hardness AH between the heat affected and mother metal zones equal to or less than a value of 2 on the C scale of the Rockwell hardness (H C). In other words, that width d is not exceeding 1 mm. provides the uniformly photoetching property regardless of the value of the hardness difference AH. Alternatively the hardness difference AH not exceeding 2 on the Rockwell hardness C scale provide the uniformly photoetchin-g property regardless of the value of the width d. In FIG. 5, the hatched portion defined by the both coordinate axes and curves a describes the width d and/or the hardness difference AH providing the uniformly photoetching property.

As above described, the molybdenum is effective for improving the photoetching property of the present steels. In addition, the molybdenum serves to decrease a temperature from which the steels beings to be transformed to the bainitic structure to aid in improving the agehardenability and to displace the over-aging temperature to a higher value.

FIG. 6 shows a graph plotting the content of molybdenum in percent (in abscissa) against the width d in mm. (in ordinate) of that portion of the heat affected zone having a difference in hardness AH equal to or less than a Rockwell hardness C of 2 between the same and the mother metal of the weld part after having aged. The reference characters have the same meaning as those designated in FIG. 5. As shown in FIG. 6, the use of molybdenum in an amount of at least 0.1 weight percent satisfies the requirement that the heat affected zone should have a difference in hardness equal to or less than 2 on the C scale of Rockwell hardness between the same and the mother metal along with the width d of about 1 mm. or less.

FIG. 7 is a graph similar to FIG. 6 but illustrating the effect of the content of carbon upon the difference in hardness between the deposited metal and mother metal of built-up weld parts after having aged. As shown in FIG. 7, the difference in hardness between the deposited metal and mother metal has a value approximating zero for the content of carbon ranging from 0.05 to 0.15% on the basis of the total weight of the steel. In FIG. 7 it is seen that the content of carbon below 0.05% as well as that exceeding 0.15% cause an increase in the hardness difference AH on H O as shown at solid and dotted line in FIG. 7. Solid line was obtained with steel aged at 500 C. for one hour while dotted line was obtained with those aged at 500 C. for ten hours.

It is recalled that molybdenum is effective for improving the photoetching property. It has been found that this effect of the molybdenum results from its cooperation with manganese, nickel, aluminum and copper included in the respective amounts as above specified in the steel. FIG. 8 shows the quantitative relationship among nickel, aluminum and copper required for the present steel including molybdenum in an amount of 0.2% to exhibit the abovementioned difference in hardness equal to or less than 2 on the C scale of Rockwell hardness. In FIG. 8 the content of nickel in percent (in ordinate) is plotted against the content of aluminum in percent (in abscissa) with the parameter being the content of copper in percent. As seen in FIG. 8, the contents of nickel, aluminum and copper has preferably the respective lower limits of about 2.5, 0.5 and 0.7 weight percent respectively. FIG. 8 shows also the relationship between the contents of nickel and aluminum for 0.5% by weight of copper. For such a content of copper more than 3% of nickel and more than 1.5% of aluminum are required to be added to the steels which is outside of the scope of the invention.

Test pieces having diameters of 50, 100, 125 and 150 mm, respectively were prepared from an ingot of Type D steel as listed in Table I and measured to have the hardnesses remaining substantially unchanged from one to the other of the ends on the diameter, regardless of their dimensions. That is, even large masses formed of the steel Type D had a hardness substantially unchanged between the surface and the interior thereof.

In order to demonstrate the effectiveness of the invention conserning the machinability, test pieces were formed of Type E steel including sulfur according to the invention, Type Y steel previously used and Type Q steel of the invention including no sulfur (see Table I) respectively. The Type Y pieces were quenched with oil at 850 C. The resulting hardness was of 35 on the C scale of Rockwell hardness. Types D and Q of test pieces were 10 From Table II it will be seen that the present steel much increases the useful life of the tool at diiferent cutting speeds with different feeds, as compared with prior art steel.

The finished surface produced by the end mill-cutting tests had roughnesses of 10 microns and microns in H- at the cutting speeds of m./min. with the feeds of 0.06 and 0.12 mm./tooth respectively and of 22 and 24 microns in H at the cutting speeds of 33 m./min. with the feeds of 0.06 and 0.12 mm./tooth for the present steel while the corresponding values were of 34, 56, 35 and 46 for the prior art steel or AISI Type 1055 steel.

A Buehler automatic polishing machine was operated at 150 r.p.m. to compare the present and prior art steels with each other. Test pieces formed of steels to be compared were embedded in respective resinous members in the form of cylinders 25 mm. in diameter and polished by emery Nos. 120, 320 and 600 under a polishing pressure of 14 kg, diamond powder having a particle size of 6 microns under a polishing pressure of 14 kg. and alumina having a particle size of 0.05 microns under a pressure of 6 kg. respectively. Type E steel (see Table I) was compared with AISI Types H11, 4145 and 1055 steels in terms of a time interval in minutes required for each steel to be completely ground. The results of age hardened at 500 C. to 42 and 43 on the Rockwell 25 tests are listed in Table IV.

hardness C scale.

TABLE III.--GRINDABILITY OF PRESENT AND PRIOR ART STEELS The cutting test using a milling machine with no cutting coil was conducted with those test pieces so that they were downwardly slitted. One slitting saw of AISI Type M2 steel, having a depth of 0.8 mm. and a feed of 0.0174 mm. per tooth was operated at a cutting speed of m./min. on each of the test pieces until it was disabled to slit the test piece. Time intervals of 105, 10 and 60 minutes elapsed for the Types E, Y and Q test pieces respectively until the associated saws were disabled to slit them. These time intervals provide a measure of useful life of the respective saws used.

The time interval of 105 minutes proves that Type E steel with sulfur has ben much improved in machinability. It is to be noted that the addition of sulfur in a very small amount to the steel as in Type E steel does not at all adversely affect the age-hardenability and photoetching property of the resulting steel. Also even Type Q steel without a machinability improving element such as sulfur has been considerably improved in machinability as compared with conventional steel such as Type Y steel.

Also end mill-cutting tests were conducted with the present and prior art steels. The present steel was of Type B (see Table I) age hardened to an H C of 41 and the prior art steel was of AISI Type 1055 normalized and having an H C of 21. One double blade tool of AISI Type M2 steel was operated on each of the test pieces until it was disabled to cut them. A time interval for which the tool was initiated to cut the particular test piece and disabled to do so was measured along with a length of travel of the tool effected during that time interval. The results of the tests are listed in the following Table II.

TABLE II.-RESULTS OF END MILL-CUTTING TESTS From Table III it is seen that as to the grindability the present steel Type E is comparative to AISI Type 4145 steel for polishing by emery and diamond powder, although the present steel is higher in hardness than the latter.

It is noted that with alumina used, the present steel is superior in grindability to the prior art steels that are much less in hardness than the present steel.

Upon manufacturing complicated molds, the electrospark machining technique may be advantageously utilized. Test piece made of Type E steel of the invention was electro-spark machined in liquid under a pressure of 20 mm. Hg with a copper electrode while a current of 2 amperes at a voltage of volts flowed through the test piece and electrode to form a hole having a depth of 20 mm. Then the hardness was measured on the wall surface of the hole and on that portion disposed under the surface and the results thereof is illustrated in FIG. 9. In FIG. 9, a Vickers hardness is plotted in ordinate against a depth in mm. measured from the machined surface in abscissa. The reference character A designates that portion of the machined surface positioned between the mouth of the hole to a depth of 0.05 mm. and the reference character B designates that portion of the holes surface positioned over a depth range of from 0.05 to 10.0 mm. From FIG. 9, it is seen that the hardnesses in a direction perpendicular to the longitudinal axis of the hole remains substantially unchanged resulting in the facilitation of the succeeding machining.

It has been found that with the mother metal and consumable electrode made of the same type of the present steel, the argon submerged arc welding process is effective for producing the built-up weld part excellent in properties. For example, Type E steel (see Table I) was used as the mother metal and consumable electrode to produce built-up weld parts according to the argon submerged arc welding process. The resulting weld parts had a hardness such as shown in FIG. 10 wherein a Vickers hardness is plotted against a distance in mm. from the boundary between the mother metal and deposited steel.

As shown in FIG. 10, the weld part as welded included at heat affected zone less in hardness than the remaining portion thereof. However, after having age hardened at 500 C. for five hours, the heat affected zone has a hardness restored substantially to its initial value. It is noted that the heat affected zone does not cause an appreciable increase in hardness that generally occurs in build-up welding of conventional steels.

During the molding operation, a flashed portion of the particular plastic may be sandwiched between the parting surfaces for the associated mold to cause the surfaces to be slightly depressed due to the compressive plastic deformation. The resistance to that depression can be considered to be one of the most important factors for determining the useful life of the mold. Experiments were conducted with the present and prior art steels in conjunction with the compressive plastic deformation. Square test pieces in the form of plates were made of Type E steel (see Table I) of the invention and of AISI Types H13, 4145 and 1055 steels having H Cs of 43, 41, 31 and 20 respectively and polished by emery No. 1,000. Then acrylonitrile-butadien-styren(ABS), polypropylene and polyacetal resins were used to form square plates having each side 10 mm. long. A pressing plate of AISI Type H13 steel having an H C of 60 was disposed upon the resinous plate that was laid upon the test piece followed by the application of a load to the assembly thus formed. Then a mocroscope was used to measure a depth of a depression occurring on the test piece.

The results of the experiments are listed in the following Table IV.

TABLE IV.DEPRESSION OCCURRING ON TEST PIECES Initial stress in Prior art resinous plate in Inven- AISI AISI AISI Type of resin tons/cm. tion E H13 4145 1055 ABS 25 O. 020 0. 024 0. 070 0. 200 20 0. 014 0. 018 0. 044 0. 060 15 0. 001 0. 03 10 0. 001

Polypropylene. 25 0. 014 0. 032 0. 096 O. 170 20 0. 001 0. 032 15 0 0. 060 10 0 0. 006

Polyacetal 25 0. 001 0. 008 0. 076

In the Table IV, the initial stress is defined by a load in tons applied to the resinous plate and divided by the initial area in cm. of the load bearing surface of the plate. From the Table IV, it will be seen that the test piece of the invention is shallower in depression than the test pieces of the prior art steels. This offers one proof that molds formed of the present steels will increase in life as compared with the existing molds.

In order to examine the dimensional stability, the present steel Type E was subjected to a solution heat treatment at 850 C. for 30 minutes and left to be cooled in the air after which it Was age hardened at 500 C. for hours followed by air cooling. The steels thus heat treated were machined into circular test pieces having a thickness of 12.0 mm. and a diameter of 60.00 mm. Each of the test pieces were then provided with an eccentric circular hole having a diameter of 34.00 mm. so that the hole had a maximum distance between the periphery thereof and the outer periphery of the circular piece along a line passing through the centers of the test piece and hole. A gap 6.00 mm. wide was cut away from the narrowest portion of the circular piece to open the eccentric hole at the outer periphery of the circular piece.

Some of the test pieces thus machined were heated to 250 C. for seven hours and then left to be cooled in the air. After the heat cycle as above described had been 12' repeated ten times, the dimensions of the various portions of the test pieces were measured. The outside and inside diameters of the test pieces only changed by 7.0 and -1.5 microns respectively while the width of the gap and the maximum distance as above described merely changed by 1.0 and 6.0 microns. Some of the test pieces were heated at 250 C. for hours and then left to be cooled in the air. Changes in the outside and inside di ameter, the width of the gap and the maximum distance were measured to be of 7.0, 1.3, 3.5 and -6.0 microns respectively. From the foregoing it will be appreciated that the present steels are high in dimensioned stability.

After having been subject to the standard heat treatment as above described in conjunction with the formation of the test pieces for the dimensional stability, test pieces of Type E steel of the invention were heated at different temperatures for 250 hours and their hardnesses were measured. For control purpose, the same process was repeated with AISI Type 4145 steels tempered at 475 and 650 C. respectively and A181 Type 1064 steel normalized.

In FIG. 11, a measured value on the C scale of Rockwell hardness is plotted in ordinate against a temperature in abscissa. FIG. 11 depicts that even the heating of the present steel for a long time interval causes no decrease in hardness and that it rather tends to increase in hardness with an increase in temperature.

What is claimed is:

1. A manganese-nickel-aluminum-copper-molybdenum system age-hardenable steel for plastic molds consisting essentially of from 0.05 to 0.18% by weight of carbon, from 0.15 to 1.0% by weight of silicon, from 1.0 to 2.0% by weight of manganese, from 2.5 to 3.5% by weight of nickel, from 0.5 to 1.5% by weight of aluminum, from 0.7 to 1.7% by Weight of copper, from 0.1 to 0.4% by weight of molybdenum and the balance being iron and very small amounts of incidental impurities, said steel after having been welded being capable of aging to provide the weld part in which the deposited metal and heat affected zone can easily be subjected to uniform photoetching to the substantially same extent as does the mother metal.

2. A manganese-nickel-aluminum-copper-molybdenum system age-hardenable steel for plastic molds consisting essentially of from 0.05 to 0.18% by weight of carbon, from 0.15 to 1.0% by weight of silicon, from 1.0 to 2.0% by weight of manganese, from 2.5 to 3.5% by weight of nickel, from 0.5 to 1.5% by Weight of aluminum, from 0.7 to 1.7% by weight of copper, from 0.1 to 0.4% by weight of molybdenum, further including at least one machinability improving element selected from the group consisting of an effective amount up to 0.3% by weight of sulfur, an effective amount up to 0.4% by Weight of lead, an effective amount up to 0.5% by weight of selenium, an effective amount up to 0.3% by weight of tellurium and an effective amount up to 0.3% by weight of bismuth; the balance being iron and very small amounts of incidental impurities, said steel after having been welded being capable of aging to provide the weld part in which the deposited metal and heat affected zone can easily be subjected to uniform photoetching to the substantially same extent as does the mother metal.

3. A manganese-nickel-aluminum-copper-molybdenum system age-hardenable steel for plastic molds consisting essentially of from 0.05 to 0.18% by weight of carbon, from 0.15 to 1.0% by weight of silicon, from 1.0 to 2.0% by weight of manganese, from 2.5 to 3.5% by weight of nickel, from 0.5 to 1.5% by Weight of aluminum, from 0.7 to 1.7% by weight of copper, from 0.1 to 0.4% by weight of molybdenum, further including at least one toughness and hardenability improving element selected from the group consisting of an effective amount up to 2.5% by Weight of chromium, an effective amount up to 0.5% by weight of tungsten, an effective amount up to 0.5% by weight of cobalt, an effective amount up to 0.5% by weight of beryllium and an effective amount up to 0.1% by weight of boron; the balance being iron and very small amounts of incidental impurities, said steel after having been welded being capable of aging to provide the weld part in which the depositing metal and heat affected zone can easily be subjected to uniform photoetching to the substantially same extent as does the mother metal.

4. A manganese-nickel-aluminum-copper-molybdenum system age-hardenable steel for plastic molds consisting essentially of from 0.05 to 0.18% by weight of carbon, from 0.15 to 1.0% by weight of silicon, from 1.0 to 2.0% by weight of manganese, from 2.5 to 3.5% by Weight of nickel, from 0.5 to 1.5% by weight of aluminum, from 0.7 to 1.7% by weight of copper, from 0.1 to 0.4% by weight of molybdenum, further including at least one grain refining element selected from the group consisting of an effective amount up to 0.5% by weight of titanium, an effective amount up to 0.5 by weight of vanadium, an effective amount up to 0.3% by weight of the sum of niobium and tantalum and an effective amount up to 0.5 by Weight of zirconium; the balance being iron and very small amounts of incidental impurities, said steel after having been welded being capable of aging to provide the weld part in which the deposited metal and heat affected zone can easily be subjected to uniform photoetching to the substantially same extent as does the mother metal.

References Cited UNITED STATES PATENTS 3,110,798 11/1963 Keay 75124 3,365,343 1/1968 Vordahl 75-124 2,236,479 3/1941 Harder 75-128 P 3,097,294 7/1963 Kubil 75124 15 3,368,887 2/1968 Enis 75124 3,499,757 3/1970 Mandich 75-124 HYLAND BIZOT, Primary Examiner US. Cl. X.R. -125 

